When a submarine is sailing submerged at shallow depth, it can extend raisable masts carrying periscopes or antennas in order to observe the surface of the sea or to communicate with a distant point; it is thus able to make observations or exchange communications within the various ranges of electromagnetic radiation propagated in the atmosphere.
The same is no longer true when the submarine is submerged at great depth, because electromagnetic radiation is propagated extremely badly through seawater. It can then only receive very low frequency radio transmissions by towing an antenna floating near the surface of the water.
In the underwater medium, as soon as the distance exceeds a few meters, observations and communications are made solely by acoustic radiation. Nevertheless, in the layer of water situated near the surface the temperature varies fairly quickly with depth, and sound rays are highly curved in a vertical plane, and for this reason, among others, acoustic transmission near the surface is very uncertain.
In such circumstances, a critical situation arises for a submarine which, while sailing submerged at a depth greater than a few tens of meters in order to avoid all risk of collision with the hulls of deep-draft ships, such as tankers, wishes to surface. In the course of the last phase of rising to the surface, before emergence, when its acoustic detection is no longer effective and it is not yet possible to use the raisable masts which would permit aerial observation, there are difficulties in detecting an unforeseeable obstacle sufficiently early, having regard to the speed of movement of the submarine and possibly that of the obstacle.
The aim of the invention is in particular to provide an aerial observation means for a submarine while it is sailing for safety reasons at a depth greater than a few tens of meters.
One solution could comprise towing a float on the surface or near the surface of the sea; this float would have to be able to support a periscope mast of a height of 5 to 10 meters in order to avoid masking by waves and to have sufficient range; the mast would have to be stabilized vertically against the action of the swell; consequently, the float would be of very large dimensions incompatible with being stowed on board a submarine.
The invention proposes another solution, which is novel and unexpected.
It is based on the fact that two or three seconds are sufficient to observe the entire surface of the sea, for example with a television camera sweeping the whole horizon.